feat: add named WiiM device profiles

[zzwong]

Summary

• add backward-compatible named device profiles with --device targeting
• add device list/add/remove/use/discover and config set/unset defaultDevice
• preserve legacy defaultHost and exact precedence: --host → WIIM_HOST → --device → defaultDevice → defaultHost
• make profile persistence atomic/crash-resistant, preserve ownership/symlinks and unknown config metadata
• document discovery-to-config workflows and correct existing command safety/availability docs

Examples

wiim device discover
wiim device add living-room 192.168.1.20
wiim device use living-room
wiim --device living-room status

Compatibility

• existing configs require no migration
• top-level and nested unknown JSON metadata is retained on rewrites
• explicit target flags are rejected for discovery and commands where targeting is meaningless

Verification

• gofmt -l . clean
• go vet ./...
• go test -race ./...
• golangci-lint run ./... (0 issues)
• Linux, macOS, and Windows test packages compile

Reviews

• every implementation task passed spec and quality review
• full-diff spec review passed and quality review approved
• adversarial findings addressed: deterministic validation, copy-on-write maps, empty target handling, atomic persistence, env-independent validation, unknown-field preservation, case collisions, ownership, symlinks, and target confusion
• final adversarial re-review: CLEAN